The invention relates to a method for contacting and connecting solar cells, wherein contacting the solar cells is carried out through wire conductors, and a plurality of solar cells are combined with each other through the wire conductors so as to form the solar cell combination.
A solar cell usually consists of a substrate having a front side and a back side, wherein a contact structure is applied onto at least one of the two sides. Typically, the contact structure has a width of at least 100 μm, whereas its thickness is only approximately 10 to 15 μm. A larger width causes a decrease of efficiency due to the resulting increased shading, whereas reducing the width results in the disadvantage that the line resistance of the contact structure is increased. Furthermore, the current of the individual contact structures is fed into busbars causing further shading on the front side surface.
On the back side of the solar cell there is usually a large-area contact structure that collects the current extensively.
Connecting solar cells is generally carried out through contact ribbons that are soldered onto the busbars of the solar cell. All the current is fed through the contact ribbons. In order to keep the resistance losses as low as possible, a certain total cross-sectional area of these ribbons is required. The result of this is that there is a loss on the front side caused by the shading.
In order to create an optimal solar module it is therefore necessary that the contact structure of the solar cell and the number and dimensions of the contact ribbons are optimized in combination with each other.
A problem here is the handling and positioning of the thin wires on the solar cell. In particular the series connection of the cells causes problems since—analogous to the solder ribbons of the standard soldering process—the wires have to be brought from the front side of the first solar cell to the back side of the second solar cell. This implies also that for contacting front and back sides, the same material has to be used.
From DE 298 05 805 U1, a device for processing solar cells is known, wherein individual solar cells are interconnected with electrical connectors so as to form a string. For this purpose, the device comprises a connection strip receptacle, a soldering paste dispenser that applies the soldering paste onto the connection strips, at least one solar cell depositing rack as a soldering station, one turning device for the connection strips and one transport device from the turning device to the solar cell depositing station for depositing the connection strips onto the solar cells.
The disadvantage of this solution is that only connectors in the form of strips can be used which, moreover, have to be turned over. A solar cell comprising at least one semiconductor layer arranged on a metallic carrier and a multiplicity of contact paths arranged on the semiconductor layer is described in DE 10 2006 041 046 A1. A lateral protrusion of at least one contact path is bent onto a back side of the carrier and is arranged electrically insulated with respect to the carrier. Solar cells arranged next to each other are preferably connected to each other through strip conductors having a perforated configuration so as to enable local contacts by soldering through. This construction of the solar cell, carrying out the method for producing the solar cell, and the constructional implementation of the strip conductor shall enable to interconnect individual solar cells so as to form solar modules and to interconnect the individual solar cells as desired. The carrier is formed here as a metal band, wherein the contact paths are arranged transverse to or along a longitudinal direction of the carrier, protrude laterally beyond the carrier band, and can be used in this manner for interconnecting. Furthermore, collecting paths are arranged transverse to the longitudinal direction and transverse to the contact paths and are electrically connected to the contact paths, and the contact paths as well as the collecting paths are glued in the region of the back side of the carrier. The contact paths or collecting paths are implemented in the form of copper wire or copper band. In order to avoid an electrical connection between the contact path and the metallic carrier, insulations implemented as edge insulations are arranged along the edge. Overall, structuring and connecting individual solar cells by using perforated strip conductors is problematic, and using edge insulations involves increased manufacturing-related expenses.
A wire system for electrically contacting a solar cell, comprising a wire conductor that runs alternately between a first contacting section and a second contacting section arranged spaced apart from the first contacting section in such a manner that the wire conductor forms the wire system with a mesh-like arrangement which extends with a multitude of meshes along an extension direction, is known from DE 10 2007 022 877 A1. The wire conductor is fixed in the mesh-like arrangement through fixing means provided in addition to the wire conductor, and/or through fixing means in the form of sections of the wire conductor, which section are wound around each other. The wire conductor runs as an endless string periodically alternating back and forth along an extension direction between a first contacting section and a second contacting section. The two contacting sections are regions of the wire system, which regions are equidistantly spaced apart from each other and run along the extension direction of the wire system. In each case two solar cells arranged next to each other have solar cell contacting sections which are arranged opposing each other in the adjoining edge regions of the solar cells and which are connected to each other through the mesh-like wire conductor. Furthermore, an electrically insulating base insulator layer is provided in the solar cell contacting section of a solar cell, and an electrically insulating emitter insulator layer is provided in the adjacent solar cell so that a series connection is generated. Producing the mesh-like wire conductor is relatively complicated. A significant disadvantage of this solution is also that the efficiency is negatively influenced because the areas for contacting between front side and back side are required in two dimensions.
The strings produced as a pre-stage for the assembly of modules have the disadvantage that the cells are put on individually and are covered with tin-plated copper band pieces which cover the cells and also extend below the next cell to be put on. These copper bands are soldered with different processes. This method requires preparation of the cells with a print image as solder connection material and as strip conductor for “collecting” the free electrons and transporting to the copper bands.
The strings are picked up by handling devices, aligned and assembled so as to form modules. The assembled strings are electrically connected through cross-connectors so as to form a module. The required connections are produced successively in individual process steps resulting in long production times.
From DE 102 39 845 C1, an electrode for contacting an electrically conductive surface, in particular at least one surface of a photovoltaic element, is known. Said surface consists of an electrically insulating, optically transparent film, an adhesive layer applied onto a surface of said film, and a first group of substantially parallel, electrically conductive wires which are embedded in the adhesive layer, protrude with a portion of their surface from the adhesive layer, and, at least on the surface protruding from the adhesive layer, are coated with a layer having a low melting point, wherein the wires of the first group are electrically connected to a first contact strip.
A second group with wires running substantially parallel to each other is arranged between the transparent film and the wires of the first group, wherein the wires of the first and second groups together form a grid, and wherein the wires of the second group are electrically connected to a second contact strip. Due to the use of the film and the adhesive, this constructional configuration is very complicated. In the case of an irregular thickness of the adhesive layer, the wires protrude irregularly from said adhesive layer or can also be completely covered by the adhesive, which can result in defects. Furthermore, the film and the adhesive remain in the module; this implies relatively high demands in terms of long-term stability to be met by the adhesive and the film and therefore causes relatively high costs. Moreover, the prefabrication of electrodes from wire, optically transparent film and adhesive is technologically sophisticated.
A method for interconnecting solar cells by using prefabricated metal mesh (e.g. aluminum gage) which is contacted with the surface of the cell on the front side and the back side is known from JP 59115576 A. Under the influence of pressure and temperature, the metal mesh is connected in each case to one cell by means of a heating unit. In the case of this solution, handling is relatively complicated.
In EP 0 440 869 A1, a component is described which comprises a photosensitive semiconductor plate having a barrier structure, electrically conductive current collector contacts arranged on both sides of the semiconductor plate, and protective coatings and current dissipating electrodes arranged on both sides of the semiconductor plate. At least the electrically conductive current collector contacts arranged on the front side of the semiconductor plate are implemented in the form of electrically connected and successive sections which are in contact or not in contact with the surface of the semiconductor plate. For this purpose, the current collector contacts are made from a bent wire. This embodiment is very complicated and it can easily occur that the thin bent wire gets crushed.
Furthermore, all known assembly technologies for solar modules have reached their limits with regard to their possible cycle time and the processing of thin cell materials, which is counteracted through stringing together a plurality of machines. In general, this has a negative effect on the production costs.
Moreover, the connection materials cover an undesirably large portion of the usable silicon surface and thus deteriorate the efficiency of the solar cell.
Furthermore, it is common to apply special metal pastes (mostly silver or silver alloys) in the form of strip conductors onto the solar cells (so-called bars), e.g., by using the screen printing method, in order to ensure contacting with the wire conductors. This makes the solar cells even more expensive.